TRANSMISSION AND SPREAD OF VIRUSES 15 
possible for a bird, whose body was once infected, to retain for some 
time the power to transmit the disease. The starling is the only British 
bird which is present in such enormous numbers, which keeps together 
in such large flocks and which for preference feeds around, or even on, 
the cattle themselves. The starlings seen in the British Isles are derived 
from two sources: first the resident birds which are mainly sedentary 
and secondly the birds of continental origin, coming from Scandinavia 
and the neighbouring countries, and which spend most of the autumn 
and winter in this country. The continental birds arrive here in late 
September, in October, and early November, and leave again in 
March and early April. Bullough points out one habit of starlings 
which is of primary importance so far as the transmission of the disease 
from place to place is concerned. For seven months of the year (June 
to December) almost all British starlings leave their nesting locality 
each evening to roost communally on some wood or reed bed and 
during the whole of the time (October to March) that the continental 
starlings are present in the British Isles they also travel each day between 
their feeding grounds and those same communal roosting places. 
The journeys made are considerable, and a bird often flies a total of 
forty to fifty miles each day for this purpose alone. During these 
daily movements the virus of foot-and-mouth disease might be 
carried to the roost by some infected bird and there passed to other 
birds which, next morning, might take it away in many other 
directions. Opportunities for the spread of the virus in the roost at 
night are very great because large numbers of sleeping birds are 
packed closely together shoulder to shoulder. At certain seasons of 
the year, one roost often contains between fifty and a hundred thousand 
starlings each night. This situation of the possible contamination of 
other birds in the roosting place offers a parallel to the spread of 
paralytic rabies among vampire bats in their roosting places. 
Bullough has shown that striking correlations exist between the 
migrations, movements, and distribution of the starlings and the geo- 
graphical and monthly incidence of the disease. An attempt was also 
made to analyse the position of foot-and-mouth disease in Finland, 
Sweden, and Denmark. In the case of Sweden, where the starling is 
absent in autumn and winter, a graph was obtained which is the 
reverse of that for the British Isles where, in autumn and winter, the 
starling is most numerous. 
The case thus made for the starling as a transmitter of foot-and- 
mouth disease virus is fairly strong and would, if substantiated, 
